^Well, from what I remember, people on Earth used to think there was life on the Moon and all the planets once it was realised they were worlds; it made sense after all: the single world people knew had life, and every strange continent and island that was discovered had it, so why not other worlds too? When the moon you're looking at every night isn't a bleak grey rock, but a lush green and blue world, that assumption seems an even more likely one. So I think that as soon as people would have the knowledge that allows them to wonder whether there is life elsewhere, they'd likely rightly assume there was life on their moon.

apenpaap, that's pretty common for them, especially for some small asteroids.

What is more miraculous than that is highly-elliptical orbits, so that when planet is near the sun, it's almost habitable world, and then it's wandering on outskirts of the system with -150 C on the surface.

I don't know if the universe is supporting these in a new version, but here's something from 095 - a little Mercury planet colliding with gas giant.